IEEE Power & Energy Magazine - January/February 2014 - 81

and the coupling switch at the Pcc is closed. the green-circled
area in figure 12(a) is redrawn in figure 12(b), which shows
that the coupling switch at the Pcc is closed when the angle
difference is 2.5°. figure 12(a) also shows the instantaneous
voltage difference at the Pcc, the real power flow through
the switch, and the instantaneous current through phase a of
the coupling switch, both before and after resynchronization.
after resynchronization, the voltage difference is zero when
the phase a current is increased to 1.33 ka (peak amplitude).
the real power flow from the utility grid to the microgrid is
increased to 6.94 MW as the other local dEr units serve the
remaining 4.13 MW of the campus load. also, the master
controller resets the reference frequency of battery storage to
60 hz through secondary control and sends tertiary control
signals to building controllers to restore the load from 8.38
MW and 4.19 Mvar to 11.07 MW and 5.54 Mvar, as shown
in figure 11(b).
after resynchronization, the microgrid is connected to the
utility grid, and the master controller will procure the optimal
hourly dispatch of dEr units and building loads, taking into
consideration the available energy in the battery storage unit.
in grid-connected mode, the utility grid will set the microgrid
voltage and frequency and the primary and secondary controls
will not respond to fluctuations in campus load. figure 13
shows that after resynchronization, the generation dispatch of
the natural gas turbine is ramped down from 8 MW and 4 Mvar
to 4 MW and 2 Mvar and the battery storage unit is no longer
dispatched in response to the tertiary control signal from the
master controller. the solar Pv and wind turbine units generate 123 kW and 8 kW, respectively, and the utility serves the
remaining 6.94 MW and 3.47 Mvar of load. the dispatch of
the solar Pv and wind generation units remains fixed, as they
do not participate in providing voltage or frequency support.

Conclusions
Microgrids are as old as the first generating plant erected by
thomas Edison in new york when his company connected
wires to local houses and businesses and provided customers
with the invaluable commodity of electricity. Edison's company affected everyone's lifestyle in 1881, using the first dc
and islanded microgrid ever established. What is different
now is that the new generation of microgrids is equipped with
sophisticated control and communication devices, based on
smart grid concepts: these are able to enhance the economics
and the reliability of highly vulnerable electric power systems.
While smart grid features can also help manage distribution system outages and allow power to be restored
much more quickly than in the past, the most exciting
technology that has evolved to mitigate the whims of
Mother nature is the microgrid. Without a concerted control strategy devised by distributed microgrids, potential
on-site dEr solutions become stranded assets, going offline as the larger distribution network also shuts down
in the midst of a storm. incorporating dEr within an
islanded microgrid can provide energy services to local
january/february 2014

residents even as the larger grid awaits costly repairs and
lengthy restorations.
as the late robert W. Galvin, former chairman of the Motorola corporation once said, "Perfect power microgrids will
ensure absolute and universal availability of energy in the quantity and quality necessary to meet every consumer's needs. it is
a system that never fails the consumer." natural disasters have
further underscored the need for resilient infrastructures for
delivering vital electricity services. in essence, when it comes
to reliability, the need for microgrids can be easily justified. to
paraphrase a well-known television commercial: cost of additional generating plants, us$x; cost of microgrid development
in a reinforced infrastructure, us$y; having access to electricity on a 10 °f night or providing emergency electric service to
hospitals with thousands of intensive care unit patients on the
second week of a power outage: priceless.

Acknowledgment
this project was funded in part by u.s. department of
Energy Grant # dE-fc26-08nt02875.

For Further Reading
M. shahidehpour. (2013, Mar.). Microgrid at iit [online].
available: http://iitmicrogrid.net
M. lamonica. (2012, nov.). Microgrids keep power flowing through sandy outages, MIT Technology Review [online].
available: http://www.technologyreview.com/view/507106/
microgrids-keep-power-flowing-through-sandy-outages.
P. asmus. (2012, nov.). devastating storms make the
case for microgrids, Navigant Research Blog [online].
available: http://www.navigantresearch.com/blog/devastatingstorms-make-the-case-for-microgrids
national Electric Manufacturers association. hurricane
sandy reconstruction: rebuild the smart way. [online].
available: http://www.nema.org/Policy/documents/hurricane-sandy-reconstruction-4web.pdf
M. lamonica. (2012, oct.). smart meters help utility
speed sandy restoration, MIT Technology Review [online].
available: http://www.technologyreview.com/view/506711/
smart-meters-help-utility-speed-sandy-restoration
l. Polycarpou. (2013, May). the microgrid solution, State
of the Planet, the Earth institute of columbia university
[online]. available: http://blogs.ei.columbia.edu/2013/05/15/
the-microgrid-solution
M. roach, "hurricane sandy and the emperor's new clothes:
Microgrids as a risk mitigation strategy for extreme weather
events," White Paper, MicroGrid horizons, dec. 2012.

Biographies
Liang Che is with the illinois institute of technology, chicago.
Mohammad Khodayar is with southern Methodist university, dallas, texas.
Mohammad Shahidehpour is with the illinois institute
of technology, chicago.
p&e
ieee power & energy magazine

81


http://www.iitmicrogrid.net http://www.technologyreview.com/view/507106/ http://www.navigantresearch.com/blog/devastating http://www.nema.org/Policy/documents/hurri http://www.technologyreview.com/view/506711/ http://blogs.ei.columbia.edu/2013/05/15/

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